The present invention relates to a non-linear signal processing apparatus which is applicable to a video tape recorder or a video disk player.
In a non-linear signal processing apparatus, non-linear emphasis is made to emphasize a high frequency band of a video signal to secure a desired S/N ratio in the high frequency band, and the degree of the high frequency emphasis is lowered only when a high energy component is contained in the high frequency band in order to prevent adverse effects due to overemphasis. FIG. 1 shows an example of the non-linear emphasis circuit. Numeral 1 denotes a first connection which is a parallel connection of a capacitor 2 and a resistor 3, numeral 4 denotes a second connection in which a pair of anti-parallelly connected diodes 5 and 6 are connected in series with a capacitor 7 and this series connection is connected in parallel to a resistor 8, numeral 9 denotes an input terminal, numeral 10 denotes an output terminal and numeral 11 denotes an amplifier for adjusting a level of an output signal. The capacitors 2 and 7 have capacitances C1 and C2, respectively, and the resistors 3 and 8 have resistances R1 and R2. They have the relationship C1R1=C2R2. Since a current flowing through the anti-parallal connection of the pair of diodes 5 and 6 varies with the voltage applied thereacross, the anti-parallel connection of the diodes 5 and 6 is considered as a variable resistor having a resistance Rd which varies from a conductor to an infinite resistor depending on the voltage applied thereacross. The amplifier 11 has an amplification factor of (R1+R2)/R2, which is a reciprocal of a voltage division factor R2/(R1+R) by the resistors 3 and 8, in order to render the signal levels of the input signal the and the output signal same.
In operation, if the signal level of the input signal applied to the input terminal 9 is sufficiently low, the voltage across the anti-parallel connection of the diodes 5 and 6 is also low and little current flows therethrough. Accordingly, the internal resistance Rd is infinite. As a result, the second connection 4 is equivalent to mere resistor 8 and the circuit of FIG. 1 presents an emphasis characteristic which emphasizes the high frequency band as shown by a gain characteristic A of FIG. 2. As the signal level of the input signal increases, the voltage across the anti-parallel connection of the diodes 5 and 6 increases and the current starts to flow. If the signal level of the input signal is sufficiently high, the current flowing through the anti-parallel connection of the diodes 5 and 6 increases and the internal resistance becomes conductive. As a result, the second connection is equivalent to a parallel connection of the resistor 8 and the capacitor 7. The impedences of the second connection 4 and the first connection 1 are equal because the relationship of C1R1=C2R2 and the gain characteristic is constant as shown by B in FIG. 2.
The above operation is explained by using a Laplace transform formula which represents a continuous time system. The transfer function H(S) of the circuit of FIG. 1 is represented by C1, C2, R1, R2 and Rd as follows. ##EQU1## X, T and Td are defined as follows. EQU X=R1/R2 (2) EQU T=C1R1=C2R2 (3) EQU Td=C2Rd (4)
H(S) is represented by X, T and Td as follows. ##EQU2##
When the signal level of the input signal is sufficiently low, the resistance Rd is infinite, and Td is also infinite from the formula (4). By putting Td as infinite in the formula (6), H(S) is represented as follows. ##EQU3## The second term on the right side of the formula (7) represents a high-pass filter, and H(S) presents the emphasis characteristic which emphasizes the high frequency band.
When the signal level of the input signal is sufficiently high, the resistance Rd is conductive or zero and Td is also zero from the formula (4). By putting Td as zero in the formula (6), EQU H(S)=1 (8)
which represents a flat gain characteristic.
The non-linear emphasis of FIG. 1 emphasizes the high frequency band of the input signal when the signal level thereof is low, but as the signal level of the input signal increases, the degree of the high frequency emphasis is lowered. When it is used in a VRT or a video disk player, it improves the S/N ratio in the high frequency band while it prevents effects due to overemphasis.
However, a non-linear signal processing apparatus such as the non-linear emphasis circuit described above employs analog signal processing techniques. It is inferior to a digital IC from the standpoints of integrity and stability. It is not easy from standpoints of circuit scale and speed to realize, by a digital signal processing technique, an apparatus which non-linearly controls a frequency characteristic in accordance with the signal level of an input signal such as a video signal which spreads over a wide band.